A:The security team tries to support a stable distribution for about ONE YEAR after the next stable
distribution has been released, except when another stable distribution is released within this year.
It is not possible to support three distributions; supporting two simultaneously is already difficult enough.

Steps below to upgrade to Debian 8.

Make sure Wheezy is updated first:

# apt-get update
# apt-get upgrade

Change package repository from wheezy to jessie in /etc/apt/sources.list